Habitat fragmentation and changes in land use are currently two major drivers of biodiversity loss around the world by causing habitat loss and reducing connectivity across landscapes. These processes affect not only species diversity, but genetic structure as well. The loss of habitat and the increased isolation prevent gene flow and accelerate genetic drift, causing loss of genetic diversity and facilitating development of genetic differentiation.
The effects of habitat fragmentation and land use changes are usually studied by relating patterns of genetic diversity and differentiation to environmental factors, habitat history, landscape structure, or to a combination thereof. However, these three drivers are rarely addressed simultaneously. In addition, these studies are usually carried out in conservation-driven contexts, and therefore tend to concentrate on hyper-fragmented landscapes and on rare or endangered species. However, how habitat fragmentation and land use affect widespread species in more typical landscapes has not been fully investigated. In this thesis I address these two gaps, and do so in three study regions, allowing for generalization of the results.
I used Abax parallelepipedus, a flightless ground beetle with low dispersal power as a model species to test how environmental factors, habitat history, and landscape structure affect genetic diversity and genetic differentiation in three study regions located across Germany. This species seldom leaves wooded habitats, and rarely crosses linear barriers such as roads and railways. It is also known to be susceptible to rapid changes in genetic structure after habitat fragmentation. Nevertheless, A. parallelepipedus is widely distributed as it can inhabit a variety of woodland types in which it maintains high population densities. Although all of my study regions represent fairly typical rural landscapes for central Europe, each consisting of a complex matrix of land uses, they differ from one another in terms of environmental factors, habitat history, and landscape structure, and thus can serve as three test cases.
In the first stage of my work, I identified polymorphic microsatellite loci which could potentially be used to study genetic diversity and differentiation in A. parallelepipedus. I then developed PCR and genotyping protocols for two suites of loci, in the end selecting to use the set of 14 fully multiplexed loci for my study. After I had developed the needed study system, I genotyped over 3300 beetles from 142 study sites.
In my investigation of how environmental factors and habitat history affect genetic diversity and genetic differentiation, I found that genetic diversity was being driven by variables that could be related to population sizes rather than by habitat history. I also did not find evidence of an influence of habitat history on the genetic differentiation patterns. Although populations of A. parallelepipedus in the past were probably smaller due to deforestation, they apparently remained large enough to prevent rapid genetic drift. Thus, recolonization processes of woodlands planted after the peak of deforestation either occurred without incurring founder effects or bottlenecks, or the effects of thereof have since been erased by gene flow.
As the genetic structure found in my landscapes is driven current processes, rather than historical ones, I carried out a landscape genetics analysis of the genetic differentiation patterns found in each of my study regions, in which I examined the relationship between genetic differentiation and landscape structure. I tested whether I could find patterns of isolation by distance, isolation by resistance, or isolation by barriers in my study regions. Surprisingly, I found no effects of land use or of fragmentation. Based on the importance of population sizes found in my previous study, combined with the beetle´s known avoidance of non-wooded areas and its inability to cross roads, I conclude that although there is probably little gene flow across my study regions, large population sizes are preventing the rapid development of genetic differentiation. Models simulating the development of genetic differentiation over time in populations of different starting sizes support this conclusion.
My work highlights the importance of population sizes in determining how patterns of genetic diversity and differentiation will develop across landscapes. While emphasis has been placed in conservation contexts on the deleterious effects of fragmentation on genetic structure, this may be overstated for widespread species in typical landscapes. In such cases, large population sizes may mitigate the development of genetic differentiation and prevent loss of alleles, despite existing barriers and lack of gene flow.